Causes and Characteristics of Mirror Twins


“It’s double the giggles and double the grins , and double the trouble if one is blessed with the twins.”

90 percent of all multiple birth accounts for twins. These twins can be of two types : Identical (maternal ) and Non-Identical (fraternal) Twins. Scientifically, the former is called as monozygotic twins and the latter is called dizygotic twins.

twin mirror

What are Mirror Twins ?

Mirror Twins or Mirror Image Twins term is used for those twins which when faced each other appear to match their reflections i.e. physical features appear same but are asymmetrical in nature. For example: If one of the twins is right- handed there may be a chance the other twin is to be left handed. It might be observed that the whorls of hairs may be in opposite directions. Even the fingerprints are the mirror images and in very rare cases they might be identical too. The anatomy of the organs is also seen as mirror image in extreme cases i.e. organs in one twin remains on the usual side while in other twin these are on the opposite side.

Mirror Identical Twins: Causes and Characteristics

Causes of Mirror Twinning

Mirror twins are monozygotic twins which accounts that they were formed when single ovarian egg was fertilized by one sperm and later divides into two different embryos. This separation is rarely seen and account for about one fourth of all identical twins.

This type of twinning may occur due to late separation of fertilized egg than usual . About 7 to 12 days of fertilization the split is observed and the two identical halves develop into 2 different individuals who are genetically the same. If the fertilized egg splits after 12 days it might end up forming Conjoint Twins .


Since these are formed from single fertilized egg , the sex of the twin remains the same . The probability of having identical twins seems to be variant.

Characteristics  of  Mirror Twins

Several reversed characteristics are observed in mirror image twins :

 Moles and Birthmarks

Seen as asymmetrically in mirror twins. For example, one twin will have a birthmark on the right side of the cheek and the other will have the same birthmark of exact size on the opposite left side of the cheek.

Hand Orientation

One of the twins may be right-handed while the other would be left handed.

Hair Whorls

The mirror twins have opposite direction of hair whorls i.e. one would have swirl clockwise and the other may have counterclockwise swirl.

Freckles and Dimples

The pattern of freckles may be asymmetric and same applies for the dimples.


Eye shape and Eyebrow shape

In mirror twin case , the shape of an eye and eyebrow shape can be reversed. If right eyebrow of one twin is more thicker in one twin , the left eyebrow of  other twin may also be thicker.

Nostril Shape

The shape might be also asymmetrical in mirror twins

Mirror Twins’ Personality

The mirror twins may have opposite personalities. For example , If one twin is coward , the other might be brave enough to face any situation. One may prefer sleeping on the right side while the other twin would prefer left side.


The positioning of internal organs might be opposite in very extreme cases of mirror image twins. This condition is known Situs Inversus , where visceral like stomach , heart or even liver are seen on opposite side than their normal anatomical position.

Is There Any Test to Detect a Mirror Pair?

Well when someone gives birth to twins it becomes important to detect whether the twins are identical or fraternal and if they are identical they are mirror image twins or not.

Because of the reason that identical twins share the same placenta, examination of the placenta just after the birth of the twins can sometimes be used to determine whether the twins are identical or fraternal. But the conformation about the identical and fraternal twins can only be given after genetic testing. This genetic testing can be done by taking a cheek swab from both the babies and testing is done and the matching of the genetic profile reveals whether the babies are identical or not.


This test based on genetics helps us only to distinguish between identical and fraternal twins, and the doubt “Is the twins are mirror image twins” still is unclear because as mirror image twins are one of the types of identical twins they ultimately share the same genetic property.

Therefore, instead of genetic examination we have to take a look at their different features and characteristics. If they consistently fall opposite for every feature then they are seem to be mirror image twins.

Difference Between Mirror Twins and Identical Twins

Identical twins are monozygotic twins which means they form from a single zygote. A zygote is formed when sperm fertilizes the egg and now this fertilized egg when splits and forms two embryos further develops into identical twins.

After fertilization if  the fertilized egg splits into two between day 2 and day 8 then identical twins develops but if this splitting takes place between day 9 and day 12 then the twins born will be the mirror image twins. If this splitting of the fertilized egg takes more than  12 days then there will be development of the conjoined twins , where the twins are not separated from each other and they may even share the same organs.

Mirror twins have their characteristics mirror to each other and they are one of the types of identical twins. All mirror twins are identical twins, but all identical twins are not the mirror twins.



The above whole article explains that the mirror twins are twins which are genetically similar to each other as they are the result of same zygote but their physical characteristics are just mirror image or we can say opposite to each other and are one of the types of the identical twins.

Hope this article was of help for all our parents!! Please share your comments/queries/tips with us and help us create a world full of Happy and Healthy Babies!!